New evidence from University of
Texas at Austin researchers posit that the great Mississippi's natural ability
to chemically filter out nitrates is being overwhelmed. UT's hydrologists demonstrate
the enormity of the filtering process for almost every drop of water that
enters into the 311,000-mile long course ending in the Gulf of Mexico.

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The
study found that 99.6% of the water is filtered through bank sediments in the
watershed's creeks, streams and rivers. While the intensity of chemical
filtration seems like a good thing, the reality is that the river's natural
filtration systems for nitrates, which rob the water of oxygen, resulting in
algal blooms creating dead zones, appear to be operating at or very close to
full capacity. Researchers believe that it is unlikely that the river’s natural
system can accommodate the high levels of nitrates that have made their way
from the contributing farmlands and communities into the watershed.

As a result of its filtration systems being overwhelmed, the river
system operates less as a buffer and more as a conveyor belt, transporting
nitrates to the Gulf of Mexico. The amount of nitrates flowing into the gulf
from the Mississippi has already created the world's second biggest dead zone,
an oxygen-depleted area where fish and other aquatic life can't survive.

The research, conducted by hydrologist Bayani Cardenas and Brian Kiel,
a Ph.D. candidate in geology, provides valuable information to those who manage
water quality efforts, including the tracking of nitrogen fertilizers used to
grow crops in the Midwest, in the Mississippi River watershed.

Aaron Packman, Civil and Environmental Engineering professor at
Northwestern University says, "There's been a lot of work to understand
surface-groundwater exchange; this is the first work putting together a
physics-based estimate on the scale of one of these big rivers, looking at the
net effect of nitrate removal in big river systems."

The Mississippi River network includes the Ohio River watershed to the
east, the Missouri River watershed to the west and the Mississippi watershed in
the middle.

Cardenas and
Kiel analyzed the waterways for sinuosity (how much they bend and curve); waterway texture materials; the time spent in the sediment
(known as the hyporheic zone); and water flow rates through
the sediment with the help of ground level data from the USGS and the EPA.

Operating as a chemical filter, microbes in the sand, gravel and mud
gobble up compounds such as oxygen and nitrates from the water before the water
discharging it back. The more time water spends in the sediment, the more some
of these compounds are transformed.